This is irrelevant. The author is only making a claim about what characteristics make pieces more likely to be fake, a claim for which it doesn’t matter whether certain types of vendors are trying to ensure authenticity.

The sizes of different pieces sold as amber are irrelevant to the author’s claim about what kinds of pieces are more likely to be fake based on the appearance of the insects inside.

The relative price of amber based on what type of life form it contains is irrelevant; the author is only making a claim about what kinds of pieces sold as insect-containing amber are more likely to be fake.

This is irrelevant, since the author is not discussing the authenticity of pieces sold as amber based on the amber material itself, but rather based on the appearance of the insects embedded in pieces sold as amber.

This strengthens the argument by affirming the assumption that real pieces of amber are not likely to contain normal-looking insects, because they are instead likely to contain awkward- or grotesque-looking insects (if they contain insects at all).